phalloides's Identifications

Codey Marchesi caught this fish in a lake in metropolitan Adelaide.

I think it is most likely to be the Barcoo Grunter (Scortum barcoo), also known as a Black Bream (like so many other fish!) - and being caught in a lake in Adelaide metro is very odd. As others have suggested, this could be an aquarium release, and is most unlikely to be a native location for them. Their normal habitat is muddy rivers and waterholes.

The pale stripe under the eye, small scales and presence of dark blotches are all features that lean to that identity. The black botches can be turned on and off at will.

Their alternative name "Jade Perch" is not a "common" name, because it is a commercial name to help market aquaculture product. Barcoo Grunter grow to 380mm and 1.5kg, and have been recorded in localities in SA, NSW, Queensland and the NT.